1. Field
This disclosure is generally related to solar cells. More specifically, this disclosure is related to a solar cell that includes a metal grid fabricated by an electroplating technique.
2. Related Art
The negative environmental impact caused by the use of fossil fuels and their rising cost have resulted in a dire need for cleaner, cheaper alternative energy sources. Among different forms of alternative energy sources, solar power has been favored for its cleanness and wide availability.
A solar cell converts light into electricity using the photovoltaic effect. There are several basic solar cell structures, including a single p-n junction solar cell, a p-i-n/n-i-p solar cell, and a multi-junction solar cell. A typical single p-n junction structure includes a p-type doped layer and an n-type doped layer. Solar cells with a single p-n junction can be homojunction solar cells or heterojunction solar cells. If both the p-doped and n-doped layers are made of similar materials (materials with equal bandgaps), the solar cell is called a homojunction solar cell. In contrast, a heterojunction solar cell includes at least two layers of materials of different bandgaps. A p-i-n/n-i-p structure includes a p-type doped layer, an n-type doped layer, and an intrinsic (undoped) semiconductor layer (the i-layer) sandwiched between the p-layer and the n-layer. A multi-junction structure includes multiple single-junction structures of different bandgaps stacked on top of one another.
In a solar cell, light is absorbed near the p-n junction, generating carriers. The carriers diffuse into the p-n junction and are separated by the built-in electric field, thus producing an electrical current across the device and external circuitry. An important metric in determining a solar cell's quality is its energy-conversion efficiency, which is defined as the ratio between power converted (from absorbed light to electrical energy) and power collected when the solar cell is connected to an electrical circuit.
FIG. 1 presents a diagram illustrating an exemplary homojunction solar cell based on a crystalline-Si (c-Si) substrate (prior art). Solar cell 100 includes a front-side Ag electrode grid 102, an anti-reflection layer 104, an emitter layer 106, a substrate 108, and an aluminum (Al) back-side electrode 110. Arrows in FIG. 1 indicate incident sunlight.
In conventional c-Si based solar cells, the current is collected by front-side Ag grid 102. To form Ag grid 102, conventional methods involve printing Ag paste (which often includes Ag particle, organic binder, and glass frit) onto the wafers and then firing the Ag paste at a temperature between 700° C. and 800° C. The high-temperature firing of the Ag paste ensures good contact between Ag and Si, and lowers the resistivity of the Ag lines. The resistivity of the fired Ag paste is typically between 5×10−6 and 8×10−6 ohm-cm, which is much higher than the resistivity of bulk silver.
In addition to the high series resistance, the electrode grid obtained by screen-printing Ag paste also has other disadvantages, including higher material cost, wider line width, and limited line height. As the price of silver rises, the material cost of the silver electrode has exceeded half of the processing cost for manufacturing solar cells. With the state-of-the-art printing technology, the Ag lines typically have a line width between 100 and 120 microns, and it is difficult to reduce the line width further. Although inkjet printing can result in narrower lines, inkjet printing suffers other problems, such as low productivity. The height of the Ag lines is also limited by the printing method. One print can produce Ag lines with a height that is less than 25 microns. Although multiple printing can produce lines with increased height, it also increases line width, which is undesirable for high-efficiency solar cells. Similarly, electroplating of Ag or Cu onto the printed Ag lines can increase line height at the expense of increased line width. In addition, the resistance of such Ag lines is still too high to meet the requirement of high-efficiency solar cells.
Another solution is to electroplate a Ni/Cu/Sn metal stack directly on the Si emitter. This method can produce a metal grid with lower resistance (the resistivity of plated Cu is typically between 2×10−6 and 3×10−6 ohm-cm). However, the adhesion of Ni to Si is less than ideal, and stress from the metal stack may result in peeling of the whole metal lines.